1) Field of the Invention
The invention relates to a method for measuring the tension of a moving web, in which method the tension of the moving web is measured by means of a measuring element provided with a guide surface which the moving web is guided to pass such that the web forms an air cushion of the air it carries between the web and the guide surface and in which method a variable representing the tension of the moving web is measured by means of measuring devices arranged in connection with the measuring element, at a distance from one another in the cross direction of the web.
The invention further relates to equipment for measuring the tension of a moving web, which equipment comprises a measuring element provided with a guide surface which the moving web is arranged to pass such that the moving web forms an air cushion of the air it carries between the web and the guide surface and which equipment further comprises measuring devices arranged in connection with the measuring element, at a distance from one another in the cross direction of the web for measuring a variable representing the tension of the moving web.
2) Description of Related Art
A finished paper web formed in paper manufacture is rolled at several stages of the process both in the paper machine and in after-treatment units or finishing systems. The paper produced in a paper machine, for example, is rolled at the end of the process in a reeler into large machine rolls of even several meters in diameter. Web tension which is too low may cause fluttering or bag-like hanging of the web, which may further lead to accumulation and even breaking of the web. Also web tension which is too high causes web breaks, even in the middle of steady run, and therefore the right tension is found between these two extremes. In a paper machine the tension applied to the web is adjusted by changing the differences in speed between the different operational groups of the machine. These different operational groups include a wire section, a press section, a drier section, which is typically divided further into several different operational groups, a calender, and a reeler.
Web tension has conventionally been measured using guide rolls provided with weighing sensors. A problem with this solution is, however, that in the measurement there is always a contact with the web, whereby the measurement may leave marks on the web or cause other damage to it. Further problems include the crawl of the zero point of the measurement and the sensitivity of the measurement to temperature variations and vibration.
The tension of a moving web is currently measured by means of a tension measuring beam having a measuring bar provided with a curved guide surface. When the moving web is guided to pass the guide surface, the air carried by the web forms an air cushion between the web and the guide surface, the pressure of the air cushion being proportional to the tension of the web. On the guide surface of the measuring bar, in the cross direction of the web and in the longitudinal direction of the tension measuring beam, there are measurement orifices into which pressure hoses are fitted through which the pressure of the air cushion between the moving web and the measuring bar is supplied into a measurement cabinet arranged in the vicinity of the measuring beam. The measurement cabinet comprises pressure sensors for determining the pressure at a measurement point corresponding to each pressure hose. Since the tension of the web is proportional to the pressure of the air cushion between the web and the measuring bar, the tension of the web can be determined at the measurement orifices on the basis of the measured pressure. Publication FI80522 discloses a solution of the above type for measuring the tension of a moving web.
A weakness in solutions relying on a tension measuring beam is that they allow the tension of a moving web to be only determined at those points in the cross direction of the web where the measuring devices arranged to the measuring bar for measuring the pressure of the air cushion are placed. Since the tension profile of the web cannot be measured either on the entire width of the web or even on a part of the width of the web, errors may appear in the measurement results and therefore the tension applied to the web cannot be adjusted with sufficient precision to optimise the runnability of the web. The resolution of the web tension measurement can naturally be improved by adding more measuring devices to the measuring bar for measuring a variable representing the web tension. However a problem arises here from the need for space for the measuring devices in the measuring bar and the considerable increase in the costs of purchasing and mounting the measuring devices.
It is an object of the present invention to provide a new solution for measuring web tension.
The method of the invention is characterized by: moving the measuring element back and forth in the cross direction of the web such that the tension profile of the web is measured at least on a part of the web width by means of the measuring devices arranged in connection with the measuring element.
The equipment of the invention, in turn, is characterized in that the equipment further comprises a device which is arranged to move the measuring element back and forth in the cross direction of the web such that the tension profile of the web is arranged to be measured at least on a part of the web width by means of the measuring devices arranged in connection with the measuring element.
The basic idea of the invention is to measure the tension of the moving web by means of a measuring element provided with a guide surface which the moving web is guided to pass such that the web forms an air cushion of the air it carries between the web and the guide surface, the measuring element comprising measuring devices arranged at a distance from one another in the cross direction of the web for measuring a variable representing the tension of the moving web. A further basic idea of the invention is that the measuring element is moved back and forth in the cross direction of the web such that the profile of the variable representing the tension of the web is measured at least on a part of the web width by means of the measuring devices arranged in connection with the measuring element. According to an embodiment of the invention, the measuring element is moved back and forth in the cross direction of the web such that the profile of the variable representing the tension of the web is measured on the entire width of the web by means of the measuring devices arranged in connection with the measuring element. According to a second embodiment of the invention, the measuring element is a tension measuring beam having a frame and a measuring bar, the measuring bar being provided with a guide surface facing the surface of the web. According to a third embodiment of the invention, the position of the measuring element is measured relative to supports of the measuring element. According to a fourth embodiment of the invention, the variable representing the tension of the web is the pressure of the air cushion between the web and the guide surface of the measuring element.
An advantage of the invention is that by moving the measuring element back and forth in the cross direction of the web, the tension profile of the moving web can be determined with accuracy at least on the part of the web width, or on the entire web width, without a need to increase the number of measuring devices arranged to the measuring element. When the measuring element that is moved back and forth is the measuring beam, the solution can be implemented with extremely small changes in the structure of the tension measuring beam and the related supports and measuring devices and the control and data processing units involved. When the position of the measuring element is measured in relation to the supports, each measurement data element representing web tension can be focused with precision on a specific point in the cross direction of the web. A preferred variable for the web tension is the pressure of the air cushion between the web and the guide surface of the measuring element, because then even very small changes in the web tension can be easily detected as changes in the air cushion pressure.
In this specification, the term xe2x80x98paperxe2x80x99 covers not only paper but also paperboard and tissue.